1. Field of the Invention
The present invention relates to antenna devices and communication terminal devices for use in HF band or UHF band telecommunication systems.
2. Description of the Related Art
Typically, in a 13.56 MHz band RFID system such as near field communication (NFC) and the like implemented in cellular phone terminals, a RFID IC chip and matching elements are mostly mounted on a printed wiring board, an antenna is pasted on an inner side surface of a terminal casing, and the IC chip is electrically (galvanically) connected to the antenna with a spring pin or the like.
On the other hand, recent radio communication terminals such as cellular phone terminals are made thinner, and in more cases, the terminal casing is “metalized” by performing, for example, magnesium plating processing on the terminal casing to compensate insufficient strength due to the thinner structure.
However, when the terminal casing is “metalized”, the metal shields off an electromagnetic field around the antenna incorporated within the terminal. This creates a problem such that communication with a counterpart antenna becomes difficult to perform.
In view of the above, Japanese Unexamined Patent Application Publication No. 2011-97657 proposes an antenna device configured such that a metal plate having a larger area than an antenna coil is disposed in proximity (magnetic coupling) with the antenna coil and utilized as a radiator.
The antenna configuration described in Japanese Unexamined Patent Application Publication No. 2011-97657 enables communication with a counterpart antenna even when the antenna is shielded by the metal. However, it is necessary to consider a decrease in mechanical strength due to formation of a slit or opening portion in the metal plate. This costs more man-hours in production. Particularly, when a slit or opening is formed in a metal casing, there may be an additional constraint on casing design. Further, since a portion close to the slit or opening portion may not be connected to a circuit ground, a partial electric potential variation may occur at the metal plate. This creates a problem such that an electric field shielding effect by the metal plate may be weakened, or a concern such that a first conductor plane and a second conductor plane may interfere with another high frequency circuit.